The decrease in sense-of-doom does seem like evidence for this. However:
Whatever my one vulnerability is, I will fake a different one. For example, ordering all mirrors removed from the palace, screaming and flinching whenever someone accidentally holds up a mirror, etc. In the climax when the hero whips out a mirror and thrusts it at my face, my reaction will be “Hmm...I think I need a shave.”
The decrease in sense-of-doom does seem like evidence for this. However:
The other thing is, quirrel...is passed out. remember the azkaban fight? it decreased a bit when quirrel passed out.
it is, at best, rather weak evidence for a truly fundamental change in Quirrelmort’s view. enough to tip the scale over the 50% mark? maybe.